On the measurement of speed of a vacuum pump

Summary After pointing out the drawbacks of the conventional procedures of pumping speed measurement, an improved procedure is described in which the admitted gas is uniformly distributed over a cross section of a tube of the same size as the pump casing. The pressure is measured above this section, where the gas is in a static state. The actual as well as the intrinsic speed of the pump can accurately be determined. As the speed factor for indicating the pumping efficiency the ratio of these speeds is suggested. The intrinsic speed is a function of the intake pressure. The highest value of this speed was found to be 65 per cent of the conductance of a well designed pump. These results do not agree with the conclusions of the diffusion theory. Correction of the Langmuir formula suggested previously by the writer is confirmed.